问题
With a date field I can do this:
ORDER BY ABS(expiry - CURRENT_DATE)
With a timestamp field I get the following error:
function abs(interval) does not exist
回答1:
Use now() or CURRENT_TIMESTAMP for the purpose.
The reason for the different outcome of your queries is this:
When you subtract two values of type date, the result is an integer and abs() is applicable.
When you subtract two values of type timestamp (or just one is a timestamp), the result is an interval, and abs() is not applicable. You could substitute with a CASE expression:
ORDER BY CASE WHEN expiry > now() THEN expiry - now() ELSE now() - expiry END
Or you can extract() the unix epoch from the resulting interval like @Craig already demonstrated. I quote: "for interval values, the total number of seconds in the interval". Then you can use abs() again:
ORDER BY abs(extract(epoch from (expiry - now())));
age() would just add a more human readable representation to the interval by summing up days into months and years for for bigger intervals. But that's beside the point: the value is only used for sorting.
As your column is of type timestamp, you should use CURRENT_TIMESTAMP (or now()) instead of CURRENT_DATE, or you will get inaccurate results (or even incorrect for "today").
回答2:
Compare with current_timestamp
SELECT the_timestamp > current_timestamp;
The age function is probably what you want when comparing them:
SELECT age(the_timestamp);
eg:
regress=# SELECT age(TIMESTAMP '2012-01-01 00:00:00');
age
----------------
8 mons 17 days
(1 row)
If you want an absolute distance, use:
SELECT abs( extract(epoch from age(the_timestamp)) );
回答3:
This works (and gives the correct sorting):
ABS(EXTRACT(DAY FROM expiry - CURRENT_TIMESTAMP))
Unfortunately, as Erwin Brandstetter pointed out, it reduces the granularity of the sorting to a full day.
来源：https://stackoverflow.com/questions/12460797/sort-timestamps-including-future-by-absolute-distance-from-now